This invention relates to a hand tool for precisely scribing a circumferential line around a tubular workpiece at a precise distance from the end of the workpiece.
The invention finds particular application for scribing a circumferential line about a pipe tubular element that is to be socket welded for the purpose of permitting inspection of the socket weld after the welding operation has been completed.
A socket weld is a welded joint between coaxial tubular workpieces, and specifically between a smaller tube disposed within a larger tube having a female socket within which the end of the smaller tube fits. In preparation for welding operation, the smaller tube is inserted within the socket of the larger tube end until the end of the smaller tube abuts the bottom of the socket. The smaller tube is then withdrawn a small distance and the welding is carried out between the free end of the larger tube socket and the outer sidewall of the smaller tube. It is critically important that the end of the smaller tube be spaced from the bottom of the socket of the larger tube, otherwise expansion forces could ultimately result in a defective weld when the workpieces expand or contract after the welding operation. It will be appreciated that it is important to maintain a small gap between the end of the smaller tube and the bottom of the socket to permit normal expansion and contraction of the free end of the smaller tube disposed within the socket. If this gap is not provided at the time of performing the weld, the resultant joint is considered to be defective and will not pass a subsequent inspection if the inspector can determine that this gap has not been provided.
The problem in the prior art has been that it is virtually impossible for an inspector to precisely determine whether or not a suitable gap has been maintained between the end of the smaller tube and the bottom of the socket of the larger tube during the welding operation, since there is no normal reference point on the outside of the smaller tube that would enable him to gauge the maximum distance remaining to the longest point on the smaller tube. The problem is compounded when it is realized that the ends of the smaller tubes are not always cut squarely, so that there may be a discontinuous or irregular gap between the end of the smaller tube and the bottom of the socket. Thus, even though a gap might be provided around a portion of the circumference of the end of the smaller tube, the longest portion of the end might actually abut the bottom of the socket, resulting in a defective joint connection.
It has thus been recognized that a device for marking the end of a smaller tube to be inserted into the socket of a larger tube end in a precise manner to provide an inspector of a weld joint with a visual means for determining the precise distance between the observable mark on the smaller tube and the maximum length of the tube is highly desirable. However, prior art devices for accomplishing such marking are not readily available for use in an environment where tube welding operations are being carried out. Such a marking device would need to be extremely rugged while yet able to perform a precise scribing operation for providing a permanent visual mark about the circumference of a tube at a precise distance from its longest dimension. Such a device would need to be relatively simple and inexpensive to manufacture since, above all, would need to be a simple hand tool readily usable by welders or helpers. The tool, moreover, would need to be readily serviceable in the field if the need should arise.
The present invention has been made with all these considerations in mind to fulfill the above objectives.